Accoustical apparatus and method for measuring water level in a ground water well having obstructions

ABSTRACT

An acoustic distance measuring device adapted to measure the level of water in a ground water well by first emitting a sonic low frequency acoustic pulse and measuring the time-of-flight for each reflected pulse from the surface of the water in the well. Each reflected pulse must have an amplitude greater than a threshold amplitude or it will be rejected. Based on time-of-flight measurements from the first low frequency sonic pulse reception windows are established for a second higher frequency sonic pulse. To refine the time-of-flight measurements for the second pulse only those reflected pulses whose reception times fall within the reception windows are accepted. A more accurate calculation of the distance from the sonic pulse emitter to the surface of the water is accomplished by using the refined time-of-flight measurements from the second higher frequency pulse.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of acoustic devices and methods usedfor measuring the level of water in a ground water well and morespecifically an acoustical device that relies upon an echoing sonicfrequency sound wave to reliably determine the level of water in aground water well that may contain obstructions.

2. Discussion of the Prior Art

The ability to measure the level of water in a ground water well is ofsignificant importance to those individuals who rely upon such wells fortheir drinking water. The most common requirement is to know the volumeof water in the well to determine the inventory of water available foruse and to determine appropriate usage rates. Additionally, since groundwater wells are sealed from surface water runoff and its contaminants,fluctuations in water levels in ground water wells may be indicative ofleakage into the well and subsequent ground water contamination.

Echo systems for through-air measurement of the level of fluid in acontainer are well known. Typically, such a system includes a transducerthat emits a burst of acoustic energy to locate the position of thesurface of a fluid in a container such as a storage tank. The level ofthe fluid within the storage tank is determined based upon the amount oftime required for the acoustic energy to travel from the transducer tothe surface of the fluid and back again. It has been recognized thatthere are various practical problems associated with these systems. Forexample, once such problem is the generation of multiple return signalscaused by reflections from the walls of the containers and possibleinternal structures such as pipes. Multiple return signals will serve toconfound the return signal detector as to which return signal isrepresentative of the true fluid level. One attempt to overcome themultiple return signal problem is described in U.S. Pat. No. 5,131,271“Ultrasonic Level Detector” issued to Haynes and Margison on Jul. 21,1992. This patent discloses a device having various improved signalprocessing features that ignores the spurious signals caused by echoesby only accepting an echo having a signal strength exceeding a magnitudeexceeding a specified threshold.

The device taught by Haynes and Margison is a system that usesultrasonic energy and therefore is suitable only for relatively shortdistances between the transducer and the level of the fluid. Fordistances involved in ground water wells, which may exceed severalhundred meters, ultrasonic energy is not suitable because of theattenuation of the ultrasonic signal over distance. Ultrasonic energy isreadily absorbed and reflected by obstructions in a ground water wellsuch as roots, ropes, pipes and wires. Ultrasonic energy has theadvantage over short distances of having a faster ramp-up rate resultingin a cleaner pulse of emitted energy. However, over a long distance, anultrasonic pulse will be reflected by a number of surfaces. Theresulting return echo ends up being a jumble of signals which aredifficult to segregate between true and false level indications. As aresult, ultrasonic energy systems such as that disclosed by the Haynesand Margison system rely upon a sophisticated software program that addsto the expense of the system and may place it out of the reach ofindividuals reliant upon ground water wells for their drinking water.

What is required is an acoustic measuring system that is simple andinexpensive to acquire and operate and overcomes the problems associatedwith spurious echoes in an environment where the attenuationcharacteristics of ultrasonic signals preclude their use.

OBJECTS OF THE INVENTION

It is an object of the present invention to overcome the deficienciesnoted in the prior art.

It is a further object of the present invention to provide an improvedapparatus and method for measuring the level of water in a ground waterwell.

It is another objective of the present invention to provide an apparatusand method for measuring the level of water in a ground water well thatmay be obstructed by pipes, electrical wiring and other thingspreventing the use of line-of-sight measurement.

It is yet another objective of the present invention to overcomedeficiencies associated with the use of ultrasonic distance measuringdevices in obstructed ground water wells by using sonic frequencies.

It is another objective of the present invention to provide an apparatusand method for measuring the level of water in a ground water well thatis inexpensively manufactured and easily operated by individuals who arenot skilled in the art of the invention.

SUMMARY OF THE INVENTION

The present invention comprises an apparatus and method for measuringthe level of water in ground water wells. The apparatus comprises asonic pulse emitter adapted to emit a sonic pulse down the ground waterwell. The apparatus further comprises a sonic pulse receiver adapted toreceive the sonic pulse reflected from the surface of water in theground water well. An excitation circuit is provided and coupled to thesonic emitter so that sonic emissions of a predetermined frequency andamplitude can be emitted. A microprocessor is coupled to the sonicreceiver and emitter so that time-of-flight (t_(f)) measurements can bemade to determine the distance from the emitter to the surface of waterin the well using the relationship D=S(t_(f))/2 where S is the speed ofsound. The apparatus further includes a programmable read only memoryfor storing an operating program and application program for themicroprocessor. The apparatus also includes a comparator circuitconnected to the sonic pulse receiver in order to compare the amplitudeof the received sonic pulse signal with a threshold amplitude valuebelow which the signal will be rejected. An analogue to digitalconverter may also be used to convert the signal level to digital formatallowing a microprocessor to perform signal analysis and to compare thesignal to a variable threshold via software routines.

In the method of the invention, low frequency emissions in the sonicrange are chosen because such emissions are not significantly attenuatedby air or humidity over long distances and are not susceptible toreflection from obstructions in the well. In one embodiment of themethod of the invention, a single low frequency pulse is emitted downthe well. To avoid constructive and destructive interference in the wellbetween emitted and reflected signals and to avoid as much spuriousechoing as possible, the ideal frequency of the sonic pulse isdetermined to be in a range between a low value and a high value. Thelow value of the frequency is calculated using the relationshipf_(LOW)=SW/2D_(MIN) where S is the speed of sound (1130 f/s or 344 m/s)and W is the mechanical vibration factor of the sonic emitter, that is,the number of times the emitter vibrates for each emission and D_(MIN)is the minimum value of the distance between the sonic emitter and thesurface of the water. The high value is calculated using therelationship f_(HIGH)=S/3θ where S is the speed of sound and θ is thediameter of the well in meters or feet. The sonic emission will bereflected up and down the well a plurality of times. These cycles ofreflection can be used to calculate an average time-of-flight andthereby enhance the accuracy of the measurement of D.

In the preferred embodiment of the method of the invention, the firstlow frequency sonic emission is used to calculate the distance D. Asecond higher frequency emission, still in the sonic range, is then usedto increase the accuracy of the first measurement. Although the higherfrequency emission will produce a reflected signal that is both noisierand weaker than that produced by the lower frequency emission, themicroprocessor can establish temporal windows located where thereflected signals from the higher frequency emission are anticipatedbased on time-of-flight measurements from the first lower frequencyemission. The frequency of the second emission is calculated using therelationship f=S/2θ where S is the speed of sound and θ is the diameterof the well in meters or feet.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention as illustratedin the accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a representation of a typical ground water well.

FIG. 2 is a representation of an initial and reflected ultrasonic pulse.

FIG. 3 is a block diagram representing the apparatus of the presentinvention.

FIG. 4 is a representation of an initial and reflected sonic pulse of300 HZ.

FIG. 5 is a representation of an initial and reflected sonic pulse of1000 HZ.

FIG. 6 is a block diagram of the operation of the invention.

FIG. 7 is a continuation of FIG. 6.

FIG. 8 is a continuation of FIG. 7.

FIG. 9 is a continuation of FIG. 8.

DETAILED DESCRIPTION

The Problem

Ground water wells are used for drinking water supplies for millions ofpeople around the world. These wells typically include metallic orconcrete casings and are usually capped with a well cap to seal thesurface of the well against surface water runoff that may becontaminated with soil bacteria, e-coli bacteria, salt, petroleum,chemicals, fecal or other contaminants which are undesirable in groundwater. Near the bottom of the well there is usually located a pump forpumping water from the well to the surface for distribution. The pump isusually electrically powered. Therefore, the ground water well will havepiping and electrical wiring running into the well. Sometimes a ropewill be attached to the pump to secure it to the surface in the event itbecomes disconnected from the pipes and wires connecting it to thesurface. A typical ground water well (10) is illustrated in FIG. 1. Theprofile of the well in FIG. 1 is greatly exaggerated in order to showthe various components and potential obstructions in the well. Suchwells may only be several inches in diameter. The well penetrates soil(12) from the surface (15) into a ground water bearing aquifer (14).Ground water (16) collects at the bottom of the well and rises to alevel (18) in the well. Level (18) varies up and down according to theseason, the amount of water drawn from the well, and weather so therewill be a D_(MIN) (29) representing the high water mark in the well andthe shortest distance the sound waves will have to travel to reflect offof the surface of the water (18). There is also a D_(MAX) (17) whichrepresents the lowest acceptable level of the water in the well and isgenerally close to the level of the pump intake. D_(MAX) may be used tocalculate an appropriate strength or amplitude for the emitted sonicpulses. The position of the surface of the water (18) is usuallymeasured as a function of the distance (D) (19) between the sonicemitter/receiver (40) located on the inside surface (43) of the well cap(22) and the surface of the water (18). An alternative position for thesonic emitter/receiver is shown as (23) on the side of the well casing(20). The distance D may be several meters or several hundred metersdepending on the location of the well and the depth of the aquifer. Thewell has an axis (33) and may include a well casing (20) which extendsthrough the soil (12) either to the vicinity of the aquifer or tobedrock, whichever is encountered first. The well casing (20) iscomprised of metal, wood, clay, concrete or some other impermeablebarrier. The well is also typically sealed with a well cap (22) toprevent surface water from running directly into the well and to preventanimals and trash from entering the well. The well typically furtherincludes an electrical motor and pump (26) having an intake (28) belowthe surface of the water and pipe (34) which attaches to a pitlessadapter elbow (41) which directs the water through the well casing (20)to the distribution system (a pressure tank or other point-of-use, notshown). The electric motor is connected to a power cable (32). Shallowwells may be operated with an external pump such as a convertible jetpump. In this case there may be no wires leading into the well, butthere may be additional pipes as a jet pump requires two pipes to enterthe well. It is also possible to operate more than one pump in a singlewell which would result in additional pipes, wires and pumps. In somecases there may be a rope (35) between the pump and the surface toensure that the pump does not fall to the bottom of the well in theevent that it becomes disconnected from the pipe (34) and electricalcable (32).

The well casing in a typical ground water well has an inside diameter(θ) (31) of about 4 to 7 inches. The diameter of the water pipe (34)bringing water to the surface of the well for distribution is typically1 to 2 inches in diameter. Therefore, such a well will have anobstruction of about 5% just from the pipe alone. Add to this, potentialobstructions from pump electrical power supply wires, additional pipes,ropes, connectors and adapters and the amount of obstruction in a groundwater well can increase significantly. Bends in the well ordiscontinuities in the well casing can also serve to obstruct the well.

Clearly, there is a need to continually monitor the position of thewater surface (18) of the water in the well. Various visual methods ofmeasurement are available such as steel tape and chalk, mechanicalpressure gauges, float and recorder systems and pressure transducer andrecorder systems. Each of these methods has inherent shortcomings. Forexample, steel tape and chalk requires multiple measurements and anaverage to be taken. Float recorders are affected by friction within therecorder and friction between the float and the sides of the well. Thesesystems can also cause water contamination as they physically contactthe water and may be moved from well to well resulting in additionalcontamination of multiple wells.

Contactless level measurement techniques offer the best solution toproblems associated with mechanical and visual systems of ground waterwell level measurements. Ultrasonic fluid level measurement techniquesare well known and are often employed in process systems. Suchtechniques utilize an apparatus comprising an ultrasonic sound waveemitter and receiver often in the form of a transducer, an excitationcircuit and a signal processor. An ultrasonic sensor is capable ofmeasuring distance by sending a pulsed ultrasound wave at the object andthen measuring the time for the sound echo to return. Knowing the speedof sound, the sensor can determine the distance of the object. There area number of advantages associated with ultrasonic distance measurementsuch as its non-contact nature, the ability to accurately measuredistances up to several meters, the fact that ultrasonic waves areunaffected by the transparency or other optical characteristics of thetarget and the fact that the surface texture of the object is generallynot a problem.

Inadequacies of Ultrasonic Measurement

For a deep ground water well, ultrasonic measurement systems areinadequate. The amount that an acoustic waveform is attenuated over agiven distance is proportional to frequency. The higher the frequency,the greater is the attenuation. Ultrasonic sound waves are quicklyattenuated over distances. For example, a 4 KHZ sound wave attenuatesapproximately 1 db for every 100 meters traveled. Depending ontemperature, pressure and humidity levels, an ultrasonic sound wave of40 KHZ may attenuate in air almost 100 db for every 100 meters traveled.As a result, for deep well measurements beyond several meters anultrasonic sound wave will not be able to create a sufficiently strongreturn echo to be useful in distance measurement applications.Additional problems are created for ultrasonic measurement techniqueswhen the ground water well contains other reflective or absorbingsurfaces in the form of obstructions between the source of theultrasonic pulse and the target, namely, the water level in the well.These obstructions are in the form of pipes, wires, and pumpingapparatus as shown in FIG. 1. Ultrasonic energy waves are easilyreflected or absorbed by such obstructions. In shallow wellapplications, obstructions between the ultrasonic source and targetwater level will create several detectable return echoes which canreduce the validity of distance measurement by confounding the returnecho detection system.

Referring to FIG. 2, there is shown a representation of an initialultrasonic pulse (40) emitted by an ultrasonic pulse emitter from thetop of a shallow ground water well as depicted in FIG. 1 down the axisof the well. The pulse has an initial amplitude A_(i) and a durationt_(i). The pulse is shown as a sinusoidal wave in FIG. 2 but a personskilled in the art would understand that the wave could be an arbitrarywaveform and could be made up of several cycles of the waveform. Theinitial pulse (42) received by the ultrasonic pulse receiver is the samepulse emitted by the emitter not having traveled the length of the welland not having reflected from any reflected surface. Since the emitterand receiver are in such close proximity, pulse (42) is not an echopulse and is received by the receiver almost instantaneously after theemitter emits the pulse (40).

Referring to FIG. 1 and FIG. 2, the next series of pulses shown are echopulses reflected from various obstructions located within the well. Forexample, echo pulse (44) could represent a reflection from the pitlessadapter (41); echo pulse (46) could represent a reflection from pipe(34); echo pulse (48) could represent a reflection from a discontinuityin the well casing (20); echo pulse (50) could represent a reflectionfrom the casing of the electric motor (26); and, echo pulse (52) couldrepresent a reflection from the target water level (18). Other pulses(54) could represent reflections caused by the signal reflecting off orbetween the obstructions listed above while returning to the surfacefrom the water level (18). Since the ultrasonic pulse emitted by theemitter would be significantly attenuated in the air in the well, echopulse (52), the furthest distant from the emitter, might appear to havea magnitude equal to or less than previous echo pulses. This wouldcreate a lot of confusion as to which signal was the desired reflectionfrom the surface of the water. Therefore, there are disadvantages andshortcomings associated with the use of an ultrasonic distance measuringdevice in a ground water well as depicted in FIG. 1.

The Apparatus of the Invention

My invention is an acoustic distance measuring device adapted to measurethe level of water in a ground water well using sonic frequencyemissions well below the ultrasonic range in order to overcome theproblems associated with ultrasonic emissions in deep and potentiallyobstructed wells previously mentioned. As shown in FIG. 1, the groundwater well has an axis (33) and a well cap (22) with an undersidesurface (43). The sonic emitter/receiver (40) is installed on theunderside surface. The position of the surface of the water in the well(18) is measured as a function of the distance (D) from the tip of thesonic emitter/receiver to the surface of the water level (18).

Referring to FIG. 3, the apparatus (300) of my invention comprises anelectrical signal generator (302) adapted to generate an electricalsignal of having an initial amplitude (A_(i)), a frequency (f) and aperiod (P). Coupled to the electrical circuit is a sonic pulse emitter(304) adapted to convert the electric signal into a sonic pulse (306).The sonic pulse emitter (304) is adapted and oriented to emit a sonicpulse (306) down the axis (33) of the ground water well. The emitted thesonic pulse (306) is reflected back (307) to the sonic pulse receiver(308) from the surface of the water (18). The sonic pulse receiver (308)is installed proximate to the sonic pulse emitter. The sonic pulsereceiver (308) is adapted and oriented to receive the reflected sonicpulse (307) having an amplitude A_(R) from the surface of the water. Thesonic pulse receiver generates an electrical signal characteristic ofthe reflected sonic pulse. The amplitude of the signal generated by thesonic pulse receiver is converted to a digital signal by an analog todigital converter circuit (310) and then received by a microprocessor(312) connected between the sonic pulse emitter and the sonic pulsereceiver. The analog to digital converter (310) may be a signalcomparator which compares the amplitude of the received signal A_(R) toa threshold A_(LIM) resulting in a one-bit representation of the analogsignal. The threshold value may be variable or fixed. Additional signalprocessing functions as are well known in the art such as filtering,phase shifting and automatic gain control can all be applied to theanalog or digital representations of the received signal. A memorydevice (314) is connected to the microprocessor and contains bothoperating programs and application programs for the microprocessor inorder to measure the time-of-flight (t_(f)) of the emitted sonic pulsefrom the sonic pulse emitter to the surface of the water in the groundwater well and back to the sonic pulse receiver. Once the time-of-flightmeasurements are made, the microprocessor calculates distance (D)according to the relationship D=S(t_(f))/2 where S is the speed ofsound. There may also be a plurality of sensors connected to themicroprocessor and located both in the well and outside of the well todetect temperature (316) and humidity (318) or precipitation (320). Theinvention may take measurements at regular intervals based on time, atintervals based on well activity such as pump operation, or based oninput directly from the operator of the invention by a manual triggerusing a remote command connection (322). The level of the water in thewell is annunciated by the communication module (324). The communicationmodule may be a screen, a wireless link, a network connection, an alarm,or a data output to another electronic device which requires informationabout the level of water in the well.

The integration level of microprocessor and microcontroller basedsystems is generally increasing. The elements shown in FIG. 3 may not bephysically implemented as discrete components, but may instead beentirely or partially integrated together in a single mixed signalcomponent.

Overcoming the Problem by Using Sonic Pulses

It is well known that low frequency pulses in the sonic range are muchless susceptible to attenuation over distance, absorption and reflectionby obstructions in their path. Therefore, there is less likelihood offalse return signals and the initial wave front of the emitted sonicpulse is more likely to reach the target water level and reflect back tothe sonic pulse receiver as a high amplitude signal easily distinguishedfrom background noise and spurious echoes. This is illustrated withreference to FIG. 4 which shows the initiation and reflection of a 300HZ pulse within a seven inch diameter well. The initial sonic pulse at300 HZ is illustrated at (60). The initial pulse is preferably a singlesine wave. However, the pulse can also be of any useful wave shape fordistance measurement. In this example, a single 300 HZ sonic pulse issent down a seven inch diameter well. The pulse (62) is not a reflectedpulse but is the emitted pulse received by the sonic pulse receiveralmost immediately after emission. Additional cycles of energy may bedetected at the sonic receiver due to mechanical vibrations of the sonicemitter and possible ringing of input filters. Small reflections at (64)and (66) may be detected due to reflections off of smaller reflectingsurfaces in the well such as the pitless adapter. However, because sonicenergy is not readily reflected off of these small surfaces, thereflected echo will appear to be very small. The sonic wave will proceedpast these obstructions generally intact and without any significantattenuation (less than 1 dB of signal loss per 100 m of travel) orreflection until it contacts the water surface in the well. The watersurface, because it is incompressible and because it completely blocksthe entire cross-section of the well, will reflect most of the incidentsonic energy back up the well to the sonic receiver. The reflectedenergy signal will appear as wave form (68) and is easilydistinguishable from the smaller wave forms representing spurious echoesand noise (70). Since the amplitude of the reflected wave is large andenergetic, it will reflect off of the inside surface (43) of the wellcap and back down the well to the surface of the water (18). The wavewill then reflect off of the surface of the water and back to the insidesurface of the well cap where it is detected as wave form (72).Subsequent reflective cycles will occur as indicated by wave forms (74)and (76) until the reflected wave no longer has an amplitude AR greaterthan the threshold amplitude A.sub.LIM of the sonic receiver. Themicroprocessor will measure the time-of-flight t.sub.f1 to t.sub.f4 foreach (successive) reflective cycle from the sonic emitter to thereflecting surface of the water in the well and back to the sonicreceiver. The microprocessor can apply standard signal processingtechniques such as averaging, deviation measurement and phase detectionto increase the accuracy of the measurement using these multiplereflections. In general, accuracy may be increased by as much as theroot of the number of samples in situations like this one. Therefore if4 echoes are detected, accuracy may be doubled when compared to a singlereflection. For 9 echoes, accuracy of the time of flight t_(f)measurements may be tripled. For the calculation of the initial time-offlight t.sub.f1 the time at which the sonic pulse is emitted is deemedto be the time at which the emitted pulse is received directly by thesonic receiver, that is, without any reflection (t.sub.0). Since thesonic emitter and sonic receiver are co-located, the time intervalbetween the sonic emission and the reception by the receiver is almostnegligible compared to the time-of-flight. The time at which thetime-of-flight ends is taken to be the time at which the reflectedsignal is received by the sonic receiver, t.sub.1. Therefore, theinitial time-of-flight is measured by the microprocessor using therelationship t.sub.f1=t.sub.1−t.sub.0. The subsequent time-of-flightmeasurements for each successive cycle are calculated in a similarfashion.

Using a First Lower Frequency Pulse and a Subsequent Second HigherFrequency Sonic Pulse

In one embodiment of the invention, an initial sonic frequency isemitted as shown in FIG. 4 and then the time-of-flights are measured anddistance D is calculated based on the reflective cycles. Distancemeasurement accuracy can be further enhanced by emitting a higherfrequency signal after the lower frequency signal is sufficientlyattenuated so as not to interfere with the subsequent higher frequencyemission. Therefore, the sonic pulse emitter and the sonic pulsereceiver are both adapted to emit and receive pulses at differentfrequencies. However, the higher frequency emission will be moresusceptible to unwanted echoes and attenuation over distances. Toovercome this problem, the first lower frequency emission as shown inFIG. 4, is used to calculate reception windows representing the temporallocation of the anticipated reflected wave, for example, t₁. When thehigher frequency emission of 1000 HZ is emitted, its reflections willfall within the range of the reception windows calculated by themicroprocessor.

Referring to FIG. 5, the initial sonic pulse (80) is emitted at 1000 HZ.The first received sonic pulse (82) is received by the sonic receiverdirectly from the emitter without reflection. The higher frequencyemission is more susceptible to being reflected from obstructions in thewell such as pipes and wires as shown in FIG. 1 as well as from thewalls of the well and the pitless adapter. Therefore, FIG. 5 shows aplurality of echo signals and noise (84). The microprocessor hascalculated the location of the first reception window (86) in the regionof t₁ based on the previous low frequency emission. The echo signal (88)falling within that window is represents the reflection from the watersurface. Echo signal (88) is reflected from the inside surface of thewell cap to the surface of the water. Again, multiple echoes (90) willbe received by the sonic receiver representing reflections fromobstructions within the well. Second reception window (92) is locatedwhere the second reflected echo signal (94) from the surface of thewater is expected to be according to the first low frequency emission.Second echo signal (94) is reflected from the inside surface of the wellcap to the water level and back to the sonic receiver. The third echosignal (96) is expected to fall within window (98) and the fourth echosignal (100) is expected to fall within window (102). Note that thehigher frequency echo signals are significantly attenuated and begin tomerge with the noise signals as the number of reflections increase.Therefore, although the higher frequency signal affords greater accuracyin distance measurement, it is necessary to establish reception windowsin order to segregate the true signal reflected from the water surfacein the well from noise. Once the amplitude of the reflected signal fallsbelow the predetermined amplitude threshold of the sonic receiver nofurther echoes are recorded. The calculation of time-of-flight for eachreflected echo and the calculation of D follows the same process asdescribed for the low frequency signal. The microprocessor can alsonarrow the windows and emit a second higher frequency pulse in order tofurther increase the accuracy of the time-of-flight calculations anddistance D measurements.

Calculating Frequencies

For the purposes of illustration of the principles of my invention, theforegoing examples used a low frequency sonic wave of 300 HZ and ahigher sonic wave of 1000 HZ. However, frequency selection is importantand is dependent upon the diameter of the well. Generally, if thewavelength of the sonic emission is less than the diameter of the wellthe sonic wave front will reflect off of the walls of the well and causea significant amount of noise received by the sonic receiver. Shortwavelengths are also prone to constructive and destructive interferenceas they travel transversely across the well and longitudinally down thewell. If the wavelength of the sonic emission is too long the wave frontwill reflect back to the sonic receiver before the actual sonic emissionis complete.

To overcome these problems, it has been determined that a the highestfirst sonic emission f_(1HIGH) should have a wavelength (λ) that is atleast three times the diameter θ of the ground water well or λ≧3θ.Therefore, f_(1HIGH) can be calculated using the relationshipf_(1HIGH)=S/3θ where S is the expected speed of sound and θ is thediameter of the well in meters or feet. For a seven inch diameter well(0.58 feet) the highest sonic frequency used for distance measurementshould not be over about 650 HZ. Similarly, the lowest frequency fordistance measurements in a seven inch diameter well f_(1LOW) can becalculated as a function of the minimum distance or depth D_(MIN) (FIG.1 Item 29) to be measured in the well f_(1LOW)=SW/2D_(MIN) where S isthe speed of sound (1130 f/s or 344 m/s) and W is the mechanicalvibration factor of the sonic emitter, that is, the number of times theemitter vibrates for each emission. For example, the sonic emitter mayoscillate 5 times for a given emission in which case W=5. If, in a seveninch diameter well, D_(MIN) is 10 feet and W=5, then f_(1LOW)=280 HZ toavoid the emitted signal from interfering with the reflected signal.Hence, a suitable low frequency emission for a seven inch ground waterwell would be about 300 HZ as per the example shown in FIG. 4. Theseformulae can be applied for very small diameter wells or bores down to 1inch in diameter as might be found in test bores. In such a case themaximum low frequency pulse would have a frequency of 4.5 kHz which iswell within the sonic range and would provide accurate measurements withlow attenuation and noise.

A second higher sonic frequency emission is used to further refine theaccuracy of the measurement D, as illustrated in FIG. 5. The secondfrequency f₂ can be calculated based on the relationship f₂=S/2θ.Therefore, for a seven inch well, the higher frequency f₂ would be about975 HZ or rounded up to 1000 HZ as illustrated in FIG. 5.

Using Multiple Pulses

Ideally, a single pulse of sonic energy having a predetermined frequencyand amplitude is required to calculate the distance D. However, in anenvironment where D is very long or there is much extraneous noise, itis desirable to emit a series of pulses [P₁ to P_(N)] in the form of ashort burst of sonic energy. The short emitted burst of sonic energymust be short enough so that it is terminated before the first reflectedsignal is received at the sonic receiver otherwise constructive [P₁ toP_(N)] or destructive interference may occur. Such a sonic burstcomprising a series of sonic pulses [P₁ to P_(N)] may be useful wherefilters are employed in the circuitry and require several receivedcycles of audio energy to properly resonate. Furthermore, a plurality ofsonic pulses is useful when the distance D to be measured is long andthe reflected signal needs to be strong enough to be detected asmultiple pulses will contain more energy than a single pulse and willtherefore be more easily detected. The number of transmitted pulses canalso be dynamically varied by the microprocessor.

Operation of the Invention

In a first mode of operation a pulse of sonic acoustic energy istransmitted down the axis of the well creating a plurality of reflectedpulses R₁ to R_(N) having respective amplitudes A_(R1) to A_(RN). Afirst reflected pulse R₁ is received by the sonic pulse receiver. Themicroprocessor will measure the time-of-flight (t_(f)) of said firstreflected pulse R₁. Then all subsequent reflected pulses R₂ to R_(N)will be received and their respective time-of-flights will be measured.Each of the individual time-of-flight measurements will be averaged forall of the plurality of reflected pulses R₁ to R_(N). Finally, themicroprocessor will calculate D using the relationship D=S(t_(favg))/2where S is the speed of sound.

It is to be understood that the comparator of the invention will compareeach of the amplitudes A_(R1) to A_(RN) of each of the plurality ofreflected pulses R₁ to R_(N) to the preset threshold amplitude A_(LIM).Values falling outside the threshold value will be ignored. Furthermore,the the highest allowable frequency f_(HIGH) of the pulse of sonicacoustic energy is determined by the relationship f_(HIGH)=S/3θ where Sis the speed of sound and θ is the inside diameter of the well and thelowest allowable frequency f_(LOW) of the pulse of sonic acoustic energyis determined by the relationship f_(LOW)=SW/2D_(MIN) wherein S is thespeed of and W is the vibration factor of the sonic pulse emitter. Thefrequency of the emitted sonic pulse will fall somewhere between f_(LOW)and F_(HIGH).

Referring now to FIGS. 6, 7, 8 and 9 the steps of the method of usingthe preferred embodiment of the invention to measure the depth of aground water well using sonic pulses will be explained. At step 200 thesoftware contained in the memory device is programmed with the insidediameter (θ) of the well and the maximum distance D_(MAX). Otherparameters such as the minimum distance D_(MIN) to be measured, thethreshold amplitude (A_(LIM)) and the emitter factor “W” representingthe number of vibrations of the emitter per emission may be determinedon a product by product basis or dynamically varied by the softwareprogram. At step 202 the mode of operation of the invention is selected,that is, the apparatus will provide periodic measurements of D at agiven time interval measured in seconds to minutes to hours. Demandmeasurements of D can be taken at any time by a remote command link(item 322 in FIG. 3) to the microprocessor. At step 202 the apparatusoperation is initiated when the mode of operation is determined. At step204 the low range of the emitted frequency (f_(1LOW)) is calculatedaccording to the relationship f_(1LOW)=SW/2D_(MIN) based upon thefactors entered in step 200. At step 206 the high range of the emittedfrequency is calculated based upon the factors entered in step 200 andusing the relationship f_(1HIGH)=S/3θ. At step 208, once the frequencyrange has been established, a suitable value for the frequency f₁between f_(1LOW) and f_(1HIGH).is selected. At step 214 corrections aremade to the speed of sound S for temperature 210 and humidity 212 inputsfrom sensors located in the well. At 216 the initial amplitude A¹ _(i)of the first emitted sonic pulse is automatically selected by theapparatus based on an inputted value of D_(MAX) so that the firstemitted pulse will have sufficient strength to reach the bottom of thewell and reflect upwards to the sonic pulse receiver. The software mayalso use a look-up table to select from a series of pre-calculated setupvalues. At step 216 the electrical signal generator generates anelectrical signal having the predetermined frequency f₁ and amplitude A¹_(i). The electrical signal is received by the sonic pulse emitter whichthen emits a first sonic pulse having the predetermined frequency f₁ andamplitude A¹ _(i). At step 218 the sonic receiver will receive theemitted pulse without any reflections and time t₀ is established. Thefirst sonic pulse will create a plurality of first reflected pulses R¹ ₁to R¹ _(N) having amplitudes A¹ _(R1-RN). At steps 220 to 231 acombination of hardware and software will continuously monitor thereceived signals from the sonic receiver. If the amplitude A¹ _(R1-N) isgreater than a certain threshold A_(LIM) then steps 226 to 231 arerepeated until no more sonic pulse reflections are detected, that is,until A¹ _(N) is less than A_(LIM). For each acceptable reflected pulsethe time t¹ _(1-N) is recorded at step 232. Once all accepted reflectedpulses have been received, time-of-flight measurements for each pulse iscalculated at 234. At step 236 the microprocessor will calculate themost accurate time-of-flight based on all accepted signals. This may bea simple average of the various time-of-flight measurements, or it mayinclude more complex algorithms. At step 238 the microprocessor willcalculate a value for D₁ based on the most accurate or averagetime-of-flight value.

In the preferred embodiment of the invention, a second higher frequencysonic pulse is emitted to improve the accuracy of the measurement of D.Based on the time-of-flight measurements previously obtained from theplurality of first reflected pulses R¹ ₁ to R¹ _(N) created by the firstsonic low frequency emission a series of reception windows WIN₁ toWIN_(N) is established for the anticipated plurality of second reflectedpulses R² ₁ to R² _(N) at step 240. Then the acceptable frequency of thesecond pulse f₂ is calculated at step 242. The second higher frequencypulse is emitted and steps 216 to 224 are repeated for the second pulse.However, to be an accepted reflected signal the reception time of thesignal must fall within the established window or be rejected at steps246 and 247. If the reception time for each subsequent reflection fallswithin the window (steps 250 to 258) then the reception time for eachsignal t² _(f1-N) is recorded. Based on these times the most accurate oraverage time-t² _(favg) of-flight calculations are rendered at 260. Atstep 262 the time-of-flight measurements are calculated. The varioustime-of-flight measurements may be averaged or a more accuratetime-of-flight may be calculated using include more complex algorithms.At 264 the microprocessor will calculate a value for D based on the mostaccurate time-of-flight value from frequency f₂. At step 266 the resultsof the calculation are sent to the communication module. Thecommunication module may be a screen, a wireless link, a networkconnection, an alarm, or a data output to another electronic devicewhich requires information about the level of water in the well.

A third or more higher frequency sonic pulses may be emitted down theaxis of the well to further refine the measurement of D using the methoddescribed above.

In another embodiment of the invention, a plurality of pulses having afrequency f₁ and f₂ can be emitted down the axis of the well instead ofa single pulse. This would be useful in the case of deep wellmeasurements of D. The duration of an acoustic emission comprising aplurality of pulses would have to be of such duration so that theemission was terminated before the first reflected pulse is returned tothe sonic receiver to avoid interference. The number of transmittedpulses could also be varied by the microprocessor.

Although this description contains much specificity, these should not beconstrued as limiting the scope of the invention by merely providingillustrations of some of the embodiment of the invention. Thus the scopeof the invention should be determined by the appended claims and theirlegal equivalents rather than by the examples given.

1. An acoustic distance measuring device adapted to measure the level ofwater in a ground water well having obstructions, said ground water wellhaving an axis, wherein said level of water is measured as a function ofthe distance (D) from said device to the surface of the water level, thedevice comprising: a. an electrical circuit adapted to generate anelectrical signal having a frequency (f); b. a sonic pulse emittercoupled to said electric circuit and adapted to convert said electricsignal into a single emitted sonic pulse having a frequency (f) and aninitial amplitude A_(i), said sonic pulse emitter co-axial with theground water well, the sonic pulse emitter adapted and oriented to emitsaid single emitted sonic pulse down said axis of the ground water wellso that the single emitted sonic pulse is reflected up and down thegroundwater well to form a series of successive reflected cyclescomprising a series of reflected sonic pulses R₁ to R_(N) havingamplitudes A_(R1) to A_(RN) to be received by; c. a sonic pulse receiverinstalled proximate to the sonic pulse emitter and internal to theground water well, said sonic pulse receiver adapted and oriented toreceive said series of reflected sonic pulses R₁ to R_(N) from thesurface of the water, the sonic pulse receiver further adapted togenerate electrical signals having strengths relative to said amplitudeA_(R1) to A_(RN); d. a comparator connected to the sonic pulse receiverfor comparing amplitudes A_(R1) to A_(RN) with a predetermined andvariable threshold value A_(LIM), said comparator adapted to reject anyreflected pulse having any of A_(R1) to A_(RN) below A_(LIM); saidcomparator adapted to send accepted signals to; e. a microprocessorconnected to said comparator and connected between the comparator andthe electrical circuit, said microprocessor programmed to calculate theaverage time-of-flight (t_(fAVG)) of said series of successive reflectedcycles having A_(R1) to A_(RN) greater than A_(LIM) and times-of-flightt₁ to t_(N) from the sonic pulse emitter to the surface of the water inthe ground water well and back to the sonic pulse receiver, themicroprocessor further adapted to calculate said distance (D) accordingto the relationship D=S(t_(fAVG))/2 where S is the speed of sound andwherein t_(fAVG)=(Σ^(t1) _(tN)/N); f. a programmable memory deviceconnected to the microprocessor for storing microprocessor operating andapplication software; g. means for remotely triggering the device from aremote location; and, h. a display for displaying the level of water ina ground water well remotely.
 2. A method of measuring the level of thesurface of water in a ground water well having obstructions using asonic acoustic pulse emitter having a vibration factor (W) and a sonicpulse receiver, said well having an axis, and an inside diameter θ,wherein said level is measured as a function of the distance D betweensaid emitter and said surface, and further wherein the surface has aminimum distance from the emitter of D_(MIN), said method comprising thesteps of: a. transmitting a plurality of pulses [P₁ to P_(N)] ofidentical frequency of sonic acoustic energy down said axis of said wellwherein said plurality of pulses [P₁ to P_(N)] of identical frequency ofsonic energy each have duration that is sufficiently short to avoidinterference with reflected pulses; b. for each of the plurality ofpulses [P₁ to P_(N)] creating a plurality of reflected pulses R₁ toR_(N) having respective amplitudes A_(R1) to A_(RN); c. receiving afirst reflected pulse R_(t); d. comparing the amplitude A_(R1) to A_(RN)of each of the plurality of reflected pulses R₁ to R_(N) to thresholdamplitude A_(LIM); e. ignoring pulses having an amplitude that is lessthan A_(LIM); f. measuring the time of flight (t_(f)) of said firstreflected pulse R₁; g. repeating steps d, e and f for each of saidplurality of reflected pulses R₁ to R_(N); h. combining saidtime-of-flight measurements (t_(fAVG)) for each of the plurality ofreflected pulses R₁ to R_(N); i. calculating D using the relationshipD=S(t_(fAVG)) where S is the speed of sound; j. repeating steps c to ifor each of the plurality of pulses [P₁ to P_(N)]; and, k. calculatingan average value D from each value of D calculated in step i for each ofthe plurality of pulses [P₁ to P_(N)].
 3. The method of claim 2 whereinthe highest allowable frequency f_(HIGH) of a pulse of sonic acousticenergy is determined by the relationship f_(HIGH)=S/3θ where S is thespeed of sound and θ is the inside diameter of the well.
 4. The methodof claim 3 wherein the lowest allowable frequency f_(LOW) of said pulseof sonic acoustic energy is determined by the relationshipf_(LOW)=SW/2D_(MIN) wherein S is the speed of sound and W is thevibration factor of the sonic pulse emitter.
 5. The method of claim 4,wherein an emission frequency f is chosen between f_(LOW) and f_(HIGH).6. A method of measuring the level of the surface of water in a groundwater well having obstructions using a sonic acoustic pulse emitterhaving a vibration factor (W) and a pulse receiver as a function of thedistance D between surface of water, wherein said well has an axis andan inside diameter (θ) said method comprising the steps of: a.transmitting a first pulse of sonic acoustic energy having a frequencyf₁ down said axis of said well thereby creating a plurality of firstreflected pulses R¹ ₁ to R¹ _(N); b. receiving at the sonic receiver afirst reflected pulse R¹ ₁ of said plurality of first reflected pulsesR¹ ₁ to R¹ _(N); c. measuring the time-of-flight (t¹ _(f1)) of saidreflected pulse R¹ ₁; d. repeating steps b and c for each of thesubsequent reflected pulses R¹ ₂ to R¹ _(N); e. recording thetime-of-flight measurements t¹ _(f2-N) for each of the plurality offirst reflected pulses R¹ ₂ to R¹ _(N); f. calculating the averagetime-of-flight measurement (t¹ _(favg)); and g. calculating a firstvalue for D₁ using the relationship D₁=S(t¹ _(favg))/2 where S is thespeed of sound.
 7. The method of claim 6 wherein said first pulse ofsonic energy comprises a plurality of first pulses of sonic energy allhaving a frequency f₁ and wherein the duration of said plurality offirst pulses is sufficiently short to avoid interference with reflectedpulses.
 8. The method as claimed in claim 6, further comprising thesteps of a. establishing a plurality receive windows WIN₁ to WIN_(N)associated with the measured time-of-flight of each of the plurality offirst reflected pulses R¹ ₁ to R¹ _(N); b. transmitting a second pulseof sonic acoustic energy having a frequency f₂ down the axis of the wellthereby creating a plurality of second reflected pulses R² ₁ to R² _(N)and wherein said second sonic pulse has a frequency f₂ higher than thefrequency of the first sonic pulse f₁; c. receiving a first reflectedpulse R² ₁ in WIN₁ of said plurality of second reflected pulses; d.rejecting any of said second reflected pulses R² ₁ to R² _(N) having atime-of-flight falling outside of WIN₁ to WIN_(N); e. recording thetime-of-flight measurements t² _(f1) for R² ₁; f. repeating steps c, dand e for each of plurality of second reflected pulses R² ₂ to R² _(N)so as to obtain t² _(f2-N); g. calculating the average time-of-flightmeasurement (t² _(favg)); and, h. calculating a second value for D₂using the relationship D₂=S(t²f_(avg))/2.
 9. The method of claim 8wherein said second pulse of sonic energy comprises a plurality ofsecond pulses of sonic energy all having a frequency f₂ and wherein theduration of said plurality of second pulses is sufficiently short toavoid interference with reflected pulses.
 10. The method of claim 9further including after step 13 b, a step wherein the amplitude of eachof the plurality of second reflected pulses R² ₁ to R² _(N) is comparedto threshold amplitude A_(LIM) and wherein second reflected pulseshaving an amplitude less than A_(LIM) are ignored.
 11. The method ofclaim 10 wherein the highest allowable frequency f_(1HIGH) of the firstsonic pulse of sonic acoustic energy is determined by the relationshipf_(1HIGH)=S/3θ where S is the speed of sound and θ is the insidediameter of the well.
 12. The method of claim 11 wherein the lowestallowable frequency f_(1LOW) of the first pulse of sonic acoustic energyis determined by the relationship f_(1LOW)=SW/2D_(MIN) wherein S is thespeed of sound and W is the vibration factor of the sonic pulse emitter.13. The method of claim 12, wherein f₁ of the first sonic pulse ischosen between f_(1LOW) and f_(1HIGH).
 14. The method of claim 13,wherein f₂ of the second pulse of sonic acoustic energy is determined bythe relationship f₂=S/2θ where S is the speed of sound and θ is thediameter of the well.
 15. An acoustic distance measuring device formeasuring the level of water in a ground water well having obstructions,wherein said well has an axis, a diameter (θ) and a water level measuredas a function of the distance (D) from said device to said water level,wherein said distance D varies between D_(MAX) and D_(MIN), and furtherwherein said acoustic distance measuring device comprises: a. anelectrical circuit adapted to generate an electrical signal having aninitial amplitude (A_(i)) and a frequency (f); b. a sonic pulse emittercoupled to said electric circuit and adapted to convert said electricsignal into a sonic pulse having a frequency (f) and an amplitude A_(i)wherein, said sonic pulse emitter is co-axial with the ground water welland has a vibration factor W, and further wherein the sonic pulseemitter is adapted and oriented to emit a sonic pulse down the axis ofthe ground water well thereby creating a series of successive reflectedcycles up and down the groundwater well comprising reflected pulsesR₁–R_(N) having amplitudes A_(R1) to A_(RN); c. a sonic pulse receiverproximate to said sonic pulse emitter, wherein said sonic pulse receiveris adapted and oriented to receive said reflected pulses R₁–R_(N) fromthe surface of the water and wherein said sonic pulse receiver isadapted to generate electrical signals having strengths relative to theamplitudes A_(R1) to A_(RN) of the reflected sonic pulses; d. acomparator circuit connected to the sonic pulse receiver for comparingthe amplitudes A_(R1) to A_(RN) of the reflected pulses R₁–R_(N) with apredetermined threshold value A_(LIM), wherein said comparator isadapted to reject any amplitudes falling below A_(LIM), and furtherwherein the comparator is adapted to convert analogue signals to digitalsignals for transmission to; e. a microprocessor connected to aprogrammable memory storing operating and application programs, whereinthe microprocessor is adapted to execute said programs to calculate theaverage time-of-flight (t_(fAVG)) of each of the successive reflectedcycles having A_(R1) to A_(RN) greater than A_(LIM) and times of flightt₁ to t_(N) from the sonic pulse emitter to the water in the groundwater well and back to the sonic pulse receiver, and wherein saidmicroprocessor is further adapted to calculate said distance (D)according to the relationship D=S(t_(fAVG))/2 where S is the speed ofsound and wherein t_(fAVG)=(Σ^(t1) _(tN)/N).
 16. A method of using anacoustic distance measuring device for measuring the level of water in aground water well having obstructions, wherein said well has an axis, adiameter (θ) and a water level measured as a function of the distance(D) from said device to said water level, and wherein said distance Dvaries between D_(MAX) and D_(MIN), and further wherein said acousticdistance measuring device comprises an electrical circuit adapted togenerate an electrical signal of having an initial amplitude (A_(i)) anda frequency (f); a sonic pulse emitter coupled to said electric circuitand adapted to convert said electric signal into a sonic pulse having afrequency (f) and an amplitude A_(i) wherein said sonic pulse emitter isco-axial with the ground water well and has a vibration factor W; andfurther wherein the sonic pulse emitter is adapted and oriented to emita sonic pulse down the axis of the ground water well thereby creating aplurality of reflected pulses R₁–R_(N); a sonic pulse receiver proximateto said sonic pulse emitter, wherein said sonic pulse receiver isadapted and oriented to receive said reflected pulses R₁–R_(N) eachhaving an amplitude A_(R1) to A_(RN) respectively, and wherein saidsonic pulse receiver is adapted to generate an electrical signal havinga strength relative to the amplitude A_(R1) to A_(RN) of the reflectedsonic pulses; a comparator circuit connected to the sonic pulse receiverfor comparing the amplitude A_(R1) to A_(RN) of the received reflectedsonic pulse with a predetermined threshold value A_(LIM), wherein saidcomparator is adapted to reject any amplitude below A_(LIM), and furtherwherein the comparator is adapted to convert analogue signals to digitalsignals for transmission to; a microprocessor connected to aprogrammable memory storing operating and application programs, whereinthe microprocessor adapted to execute said programs to calculate thetime-of-flight (t_(f)) of each of the emitted sonic pulses, and whereinsaid microprocessor is further adapted to calculate said distance (D)according to the relationship D=S(t_(f))/2 where S is the speed ofsound, said method comprising the following steps: a. programming saidapplication software by inputting values for W, θ, D_(MIN), D_(MAX) andA_(LIM); b. selecting a mode of operation; c. calculating the low rangeof a first frequency f_(1LOW) of a first sonic pulse to be emitted bythe sonic pulse emitter; d. calculating the high range of a firstfrequency f_(1HIGH) of said first sonic pulse to be emitted by the sonicpulse emitter; e. selecting a first sonic pulse frequency f₁ for thefirst sonic pulse between f_(1LOW) and f_(1HIGH); and, f. selectingamplitude A_(i) for the first sonic pulse.
 17. The method of claim 16,wherein the step of selecting the mode of operation comprises the stepof selecting one of the following modes of operation: automaticmeasurements per second; automatic measurements per minute; and,automatic measurements per hour.
 18. The method as claimed in claim 17,wherein the step of calculating f_(1LOW) of a first sonic pulse to beemitted by the sonic pulse emitter is based on the relationshipf_(1LOW)=SW/2D_(MIN) where S is the speed of sound, W is the vibrationfactor of the sonic emitter and D_(MIN) is the minimum distance to bemeasured between the inside surface of the well cap and the level ofwater in the well.
 19. The method as claimed in claim 18, wherein thestep of calculating f_(1HIGH) of said first sonic pulse to be emitted bythe sonic pulse emitter is based on the relationship f_(1HIGH)=S/3θwhere S is the speed of sound and θ is the diameter of the well.
 20. Themethod as claimed in claim 19, wherein the step of selecting a firstsonic pulse frequency f₁ for the emission between f_(1LOW) and f_(1HIGH)is accomplished by the application software.
 21. The method as claimedin claim 20, wherein the step of selecting an amplitude A for the firstsonic pulse is accomplished by the application software and is dependentupon D_(MAX) so that the emitted first sonic pulse will have sufficientstrength to reach D_(MAX) and reflect back to the sonic pulse receiver.22. The method of claim 21, further comprising the steps of: a.correcting the value for the speed of sound S for ambient conditionswithin the well; b. initiating a first excitation signal having afrequency f₁; c. emitting a first sonic pulse from the sonic pulseemitter wherein said first sonic pulse has a frequency equal to f₁ andan amplitude equal to A_(i) and wherein the first sonic pulse is adaptedto travel down the axis of the well to the surface of water in the well,reflect off of the surface and travel back to the sonic pulse receiverthereby creating a plurality of first reflected pulses R¹ ₁ to R¹ _(N)each having an amplitude A_(R1-N) respectively; d. detecting said firstsonic pulse at the sonic pulse receiver and establishing the time atwhich the emitted sonic pulse is detected as t₀; e. detecting a firstreflected pulse R¹ ₁ of said plurality of first reflected pulses whereinsaid first reflected pulse has an amplitude A¹ _(R1) and establishingthe time at which R¹ ₁ is detected as t¹ ₁; f. comparing A¹ _(R1) withA_(LIM) wherein R¹ ₁ is rejected when A¹ _(R1) is less than A_(LIM) andwherein R¹ ₁ is accepted when A¹ _(R1) is greater than or equal toA_(LIM); and, g. repeating steps e and f for reflected pulses R¹ ₂ to R¹_(N).
 23. The method of claim 22 wherein said first pulse of sonicenergy comprises a plurality of pulses of sonic energy all having afrequency f₁ and wherein the duration of said plurality of pulses issufficiently short to avoid interference with reflected pulses.
 24. Themethod of claim 23, further comprising the steps of: a. calculating thetime-of-flight t_(f) for each reflected pulse R¹ ₁ to R¹ _(N); b.calculating a combined time-of-flight t¹ _(AVG); and, c. calculating afirst value for D₁ according to the relationship D₁=S(t¹ _(AVG))/2 whereS is the speed of sound.
 25. The method of claim 24, further comprisingthe steps of: a. establishing receive windows WIN₁ to WIN_(N) based onsaid time-of-flight calculations for each reflected pulse R¹ ₁ to R¹_(N); b. calculating a second frequency f₂ for a second sonic pulsewherein said second frequency f₂ is higher than the first frequency f₁and is calculated according to the relationship f₂=S/2θ where S is thespeed of sound and θ is the diameter of the well; c. emitting saidsecond sonic pulse having frequency f₂ thereby creating a plurality ofsecond reflected pulses R² ₁ to R² _(N); d. detecting said second sonicpulse at the sonic pulse receiver and establishing t₀; e. detectingfirst reflected pulse R² ₁ of said plurality of second reflected pulsesR² ₁ to R² _(N) in WIN₁ and establishing t¹ ₁; f. accepting R² ₁ when A²_(R1) is greater than A_(LIM); g. calculating a time-of-flight t² _(f1)for R² ₁; h. repeating steps e, f and g for each subsequent reflectedpulse R² ₂ to R² _(N); i. calculating an average time-of-flight t²_(AVG); j. calculating a second value for distance D₂ based on therelationship D₂=S(t² _(AVG))/2 where S is the speed of sound; and, k.displaying the second calculated value of D₂.
 26. The method of claim 25wherein said second pulse of sonic energy comprises a plurality ofpulses of sonic energy all having a frequency f₂ and wherein theduration of said plurality of second pulses is sufficiently short toavoid interference with reflected pulses.